Among switching power supply devices, there are those that detect a current flowing through an inductor, and control an input current or output current. The kind of power factor correction circuit, or the like, that controls an alternating current input current into a sinusoidal waveform and suppress harmonic currents to an alternating current power supply, shown in Patent Document 1, is known as one example thereof.
FIG. 10 is a circuit diagram showing a heretofore known example of a power factor correction circuit that has the same configuration as the power factor correction circuit shown in Patent Document 1. In the heretofore known example, which is a boost chopper type power factor correction circuit, the output of an alternating current power supply 1 is rectified by a full-wave rectifier 3, and the output voltage of the rectifier 3 is applied to a MOSFET 5 via an inductor 4. The inductor 4 accumulates and releases energy in accordance with a turning on and off of the MOSFET 5, and supplies the released energy to a smoothing capacitor 7 via a diode 6. At this time, a voltage corresponding to the current (inductor current) flowing through the inductor 4 is generated at either end of a current detecting resistor 10.
Next, a description will be given of a control circuit 100 that controls the turning on and off of the MOSFET 5 in such a way that the power factor is corrected. The terminal voltage of the smoothing capacitor 7, that is, the direct current output voltage output from the output terminals 2a and 2b, is divided by a voltage dividing circuit formed from resistors 103 and 104. Thereupon, a voltage error amplifier 105 detects an error in the divided voltage with respect to a reference voltage 106, and outputs an error signal indicating the error.
Meanwhile, the output voltage of the rectifier 3, which is a positive voltage, is divided by a voltage dividing circuit formed from resistors 101 and 102. A multiplier 107 executes a calculation whereby the divided voltage is multiplied by the above-mentioned error signal, and outputs the result of the calculation as a current command. A current error amplifier 108 detects an error in the inductor current with respect to the current command, and outputs an error signal indicating the error. Thereupon, a PWM comparator 110 compares the error signal and a carrier signal 109, and outputs a duty ratio gate control signal corresponding to the value of the error signal.
The gate control signal is input into a gate of the MOSFET 5 via a gate driver 111. Consequently, the on-off timing of the MOSFET 5 is controlled in such a way that the inductor current coincides with the current command, as a result of which, the direct current output voltage is controlled so as to become the voltage specified by the reference voltage 106, and an average value of the inductor current is controlled to be a sinusoidal waveform. Although a phase compensation component is provided in each of the voltage error amplifier 105 and current error amplifier 108, these are omitted from FIG. 10. Also, an inverting amplifier circuit for adjusting the polarity and size of a signal input into the inverting input terminal of the current error amplifier 108 is provided between the current detecting resistor 10 and the inverting input terminal of the current error amplifier 108, but this is also omitted.
The heretofore described kind of control is called an average current control, and is advantageous in that there is little distortion in the alternating current input current, even when a discontinuous mode, in which there exists a period in which the inductor current returns to zero every switching cycle, and a continuous mode, in which the inductor current does not return to zero every switching cycle, are mixed.
However, when the inductor current is detected by the current detecting resistor 10 as heretofore described, a problem occurs in that the larger the capacity of the switching power supply device, the greater the power loss of the current detecting resistor 10, and the conversion efficiency decreases. As a countermeasure, it is possible to reduce the power loss by using a DCCT (DC current transformer) incorporating a Hall element, or the like, in place of the current detecting resistor 10, but as the DCCT is comparatively expensive, using it leads to an increase in the cost of the device.
FIG. 11 shows a heretofore known example wherein comparatively low cost ACCTs (AC current transformers) 8 and 8a are used as current detection means. In the heretofore known example, the current flowing through the MOSFET 5 is detected by the ACCT 8, the current flowing through the diode 6 is detected by the ACCT 8a, and these currents are synthesized in a current detector circuit 300a. Consequently, a signal corresponding to the current flowing through the inductor 4 is output from the current detector circuit 300a. 
FIG. 12 shows an example of a configuration of the current detector circuit 300a. The current detector circuit 300a includes a voltage limiter formed from Zener diodes 301a and 302a provided between secondary coils of the ACCT 8, a voltage limiter formed from Zener diodes 305a and 306a provided between secondary coils of the ACCT 8a, diodes 303a and 307a that rectify the output signals of the ACCTs 8 and 8a respectively, and a resistor 304a connected between a cathode connection point (signal synthesis point) of the diodes 303a and 307a and a grounding point, wherein a signal voltage corresponding to the current flowing through the inductor 4 is output from the signal synthesis point.
Meanwhile, in Patent Document 2, a technology is described whereby an inductor current of a DC/DC converter is estimated using an ACCT and a charging and discharging of a capacitor.